It is not unusual today that the ultimate quality of a scanned image is generally limited by the ability of the scanner to resolve minute features in the original object being scanned. A computer image from a scanner contains a large number of computer picture elements, or pixels. The more pixels per unit area in the image, the better its resolution and overall image quality.
High resolution scanning provides faithful reproduction of graphic art material. High resolutions require tight production tolerances for critical scanner parameters, such as the object-to-image distance, or focus, the speed and linearity of the moving platform and the color accuracy.
Scanners available today are capable of producing images at a single resolution. In other words, today's scanners usually are a compromise between scanning speed and pixel resolution. A scanner today with low pixel resolution can convert an original object to a computer image rapidly, and a scanner with high pixel resolution can convert the same original object but more slowly.
An apparatus for multiple mode scanning is described in detail hereinafter. According to one aspect of the present invention, the apparatus has a main support, a multiple transport carriage assembly, a pivoting lamp assembly and a camera box assembly.
The multiple transport carriage assembly carries the original object to be scanned. The camera box which forms the scanned image carries the pivoting lamp assembly. The main support also encloses an electronics box which contains circuitry used to control the apparatus and to interface the scanned output image to a host computer.
The scanner is fabricated with heavy-duty, generally expensive materials required by the low-tolerance, high-precision scanning requirements. An outer cover encloses the entire apparatus.
The scanning apparatus, described in detail hereinafter, is arranged to handle both opaque and transparent originals that vary in size from standard 35 mm film to large 12.times.18 inch posters. The apparatus can achieve extremely high scanning resolutions up to 4000 pixels per inch.
Scanner resolution capability is measured in pixels per inch, or ppi. The length dimension, in inches, is measured along a single scan line. The scanning resolution for a personal computer application is approximately 72 pixels per inch, compared with the 4000 pixels per inch for the present invention.
An apparatus according to the present invention, as described in detail hereinafter, contains a unique arrangement of lead screws, pulleys and shafts that control camera box positioning within a 9 micron tolerance for object-to-image distance. Only with such tight tolerances will high resolution scanning according to the present invention provide acceptable results.
Further, the camera box of the disclosed invention contains a number of individual lenses mounted in a turret assembly. Each lens provides a single scanning resolution.
When a user selects a particular resolution for scanning, the turret assembly moves to place the correct lens between the original object and the electro-optical imaging device. Precision bearings and overall construction of the multi-lens turret assembly also achieves the 9 micron tolerance requirement for an acceptable object-to-image distance.
The selected lens focuses the light energy from the original object onto a Charge Coupled Device, or CCD. The CCD is a device for converting optical signals into computer pixels at high ppi resolutions. It is a linear array of photodetectors accessed like a shift register with voltage output proportional to light level.
In this case, the CCD has 8,000 triads of photodetectors along its 72 centimeter length, giving the CCD an intrinsic resolution of 2,822 ppi in full red, green, blue (RGB) color. The lenses convert the intrinsic 2,822 ppi CCD resolution to the multiple scanner resolutions of 667, 1,000, 2,000, 3,000 and 4,000 ppi.
To achieve extremely high scanning resolutions of up to 4000 pixels per inch, the object-to-image distance must be controlled precisely to maintain optical focus. That is, a scan line on the original object and the corresponding scan line on the electro-optical imaging device must remain parallel to each other within 9 microns at all times during the scanning operation.